<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>正则表达式字符边界约束</title>
</head>
<body>
    <input type="text" name="user">
    <span></span>
    <script>
        let hd = "afd4587ert";
        console.log(/^\w+$/.test(hd));

        document.querySelector('[name="user"]').addEventListener('keyup',function(){
            let flag = this.value.match(/^[a-z]{4,6}$/);
            document.querySelector('span').innerHTML = flag ? '正确' : '错误';
        })
    </script>
</body>
</html>